1. Field of the Invention
The present invention relates to a dry toner for use in electrophotography.
2. Discussion of Background
In accordance with a dry-type electrophotographic method, latent electrostatic images are formed on a photoconductor by a conventional method, and the latent electrostatic images are developed into visible toner images with a dry toner. Then, the toner images are transferred to a sheet of copy paper and fixed thereon, for instance, by the application of heat using heat-application means such as a heated roller.
The dry toner for use with the above-mentioned electrophotographic method comprises a binder resin and a coloring agent as the main components, and when necessary, may further comprise other additives such as a charge controlling agent and an offset-preventing agent. When various characteristics required for the dry toner, namely, transparency, electrical insulating properties, water resistance, fluidity of particles, mechanical strength, glossiness, thermoplasticity, and grindability are taken into consideration, polystyrene, styrene-acrylic copolymer, polyester resin and epoxy resin are generally used as the binder agents for the dry toner. In particular, styrene-based resins are widely employed because the grindability, water resistance and fluidity are superior to others. However, in the case where toner images are formed on a sheet of copy paper using a toner comprising the styrene-based resin as the binder resin, and thereafter the toner-image-bearing sheet is held between a document folder made from a vinyl chloride resin for a long period of time, the vinyl chloride folder is stained with the toner images when the toner-image-bearing sheet is taken out of the folder. The reason for this problem is that a plasticizer contained in the vinyl chloride resin is transferred to the toner images and plasticizes the same while the toner images are brought into contact with the vinyl chloride folder. Consequently, the toner images are partially or entirely peeled from the copy paper and attached to the vinyl chloride folder. The same problem as mentioned above is produced when the toner comprising the polyester resin as the binder resin is employed.
To solve the above-mentioned problem, it is proposed to mix the styrene-based resin or polyester resin with an epoxy resin which is not plasticized by the plasticizer for use in the vinyl chloride resin, as disclosed in Japanese Laid-Open Patent Applications 60-263951 and 61-24025.
However, when different kinds of resins are contained in a toner composition, especially for a color toner, the offset properties, pigmentation, light transmission properties and coloring characteristics considerably deteriorate because of incompatibility between the different kinds of resins. Furthermore, a toner-image-bearing sheet readily curls after image-fixing process, and the glossiness of the obtained toner image decreases. The color toner images which are not provided with appropriate gloss appear to be poor in quality.
All the aforementioned problems cannot be solved by using not only any conventionally known epoxy resin, but also an acetyl-modified epoxy resin as disclosed in Japanese Laid-Open Patent Application 61-235852.
In the case where the epoxy resin is used alone as the binder resin in the dry toner, the reactivity of the epoxy resin and amine causes some problems.
More specifically, the epoxy resin is commonly used as a cured resin in such a manner that epoxy group in the epoxy resin is allowed to react with a curing agent to form a crosslinking structure, so that excellent mechanical strength and chemical resistance can be imparted to the cured epoxy resin. The above-mentioned curing agent is roughly classified into two groups, that is, an amine-containing compound and an organic acid anhydride.
When the epoxy resin serving as the thermoplastic resin is kneaded with an amine-containing dye, pigment or charge controlling agent for the preparation of a toner composition, there is the problem that the epoxy resin may cause the crosslinking reaction with such an amine-containing component in the kneading process. The toner thus obtained is not available for use in practice. Further, the epoxy resin irritates the skin because of the biochemical activity of epoxy group, so that it is necessary to handle the epoxy resin with the utmost care.
Furthermore, water absorption of the epoxy resin becomes striking under the circumstances of high temperature and humidity because of the hydrophilic nature of epoxy group. Under such circumstances, therefore, the charging characteristics of the toner are decreased, the toner deposition on the background of the photoconductor takes place, and the cleaning of the photoconductor cannot be successfully carried out. In addition, the charging stability of the toner is poor when the epoxy resin is used as the binder resin in the toner.
To prepare a toner composition, a mixture of a coloring agent such as a dye or pigment, a charge controlling agent and a binder resin is generally kneaded in a heated roll mill to uniformly disperse the coloring agent and the charge controlling agent in the binder resin. Some dyes and pigments have the charge controlling characteristics, and such dyes and pigments function both as the coloring agent and the charge controlling agent. When the epoxy resin is used as the binder resin, it is difficult to thoroughly disperse the coloring agent and the charge controlling agent in the epoxy resin. Poor dispersion of the coloring agent decreases the pigmentation and impairs the coloring characteristics of the toner. In addition, when the charge controlling agent is not sufficiently dispersed in the binder resin, the toner cannot uniformly be charged. Consequently, the charging failure occurs, the toner deposition on the background and scattering of toner particles in the copying machine easily take place, the obtained toner images are lacking in image density and evenness, and the cleaning of the photoconductor cannot be successfully carried out.
There is proposed a toner comprising as a binder resin an ester-modified epoxy resin which is prepared by reacting an epoxy resin and .epsilon.-caprolactone, as disclosed in Japanese Laid-Open Patent Application 61-219051. In this case, the transfer of the toner image formed on the copy paper toward a vinyl chloride material can be prevented, and the fluidity of toner particles can be increased. However, the amount of the ester-modified epoxy resin is as high as 15 to 90 wt. % of the entire weight of the epoxy resin, so that the softening point of the obtained toner is extremely decreased, and the obtained images become too glossy.
A positively-chargeable resin for use in the toner is proposed, as disclosed in Japanese Laid-Open Patent Application 52-86334, which resin is obtained by allowing aliphatic primary or secondary amine to react with terminal epoxy group of a conventional epoxy resin. However, the epoxy group and the amine cause the crosslinking reaction, as previously described, so that the toner thus prepared may not be suitable for use in practice.
As disclosed in Japanese Laid-Open Patent Application 52-156632, it is proposed that at least one terminal epoxy group of the epoxy resin is allowed to react with an alcohol, a phenol, a Grignard reagent, an organic acid, a sodium acetylide, and an alkyl chloride. In this case, when one of the terminal epoxy groups is not capped, the problems of the reactivity of the epoxy group with amine, the toxicity, and the hydrophilic nature remain unsolved. In addition, all of the above-mentioned reaction products of the epoxy resin are not effective as the binder resin for use in the dry toner because some of them are hydrophilic, or have an adverse effect on the charging characteristics and the grindability of the toner.
As disclosed in Japanese Laid-Open Patent Application 1-267560, a resin for use in the toner is prepared by allowing both terminal epoxy groups of an epoxy resin to react with an active-hydrogen-containing monovalent compound, and esterifying the reaction product thus obtained by use of a monocarboxylic acid or ester derivatives thereof, and a lactone. Although the problems of the reactivity with amine, the toxicity, and the hydrophilic nature, of the epoxy resin can be solved, the curling problem of the toner image after image-fixing remains unsolved.